In cellular mobile communication networks, simultaneous transmission over air interfaces using the same physical resources frequently occurs. Thereby, co-channel interference is caused which reduces the signal quality of transmission signals measured e.g. as signal to interference and noise ratio (SINR). This in turn reduces the transmission capacity of the mobile communication network. Co-channel interference becomes even more critical in mobile communication networks having a dense deployment of access nodes.
Cooperation of access nodes is a possibility to increase the signal quality of transmission signals and the transmission capacity of a mobile communication network. For access node cooperation, an initial communication between an access node (a so called “serving access node”) and a mobile station, also denoted as user equipment (UE), is extended in that the mobile station additionally communicates via at least one further access node (a so-called “supporting access node” or “cooperating access node”). Thereby, the serving access node additionally uses the transmission and receiving capacities of the at least one supporting access node for communicating with the mobile station.
For using the additional transmission and receiving capacities, user data, control data and data for operation and maintenance (O&M) has to be exchanged between the serving access node and the at least one supporting access node (e.g., via a core network). Different kinds of data may be exchanged for this purpose, for example decoded data, soft bits or I/Q data. In 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution), such an access node cooperation is denoted Coordinated Multi-Point Transmission and Reception (CoMP).
For simplicity the terms “cell” and “access node of a cell” or “base station of a cell” are generally used as synonyms throughout this text if it is clear that the term refers to the corresponding device while reference to the coverage area of the cell is generally indicated by additional terms like “cell edge”, “cell shape” or “UEs within the cell”.
In DL (Downlink, i.e. for transmissions towards a UE) CoMP of LTE, cooperating cells mitigate co-channel interference by means of coordinated scheduling, coordinated beam forming or joint transmission. Besides interference mitigating, cooperating cells transmit the same data to a UE when applying joint transmission. In order to do so the serving cell has to transfer the UE data to the cooperating cells in advance. In UL (Uplink, i.e. for transmissions from a UE) CoMP of LTE, a UE signal is received at different cells. Cooperating cells transfer the received signal, potentially after pre-processing, to the serving cell which jointly processes the received information.
In case of intra-site (intra-BS) CoMP cells or sectors of the same BS, i.e. the same site, are cooperating. The information exchange between cooperating cells is not performed through an external backhaul link but only between processing boards of the same BS (Base Station). In case of inter-site (inter-BS) CoMP cells of any BS (any site) may cooperate. The information exchange between cooperating cells is performed via a logical interface, e.g. the X2 interface for LTE, physically transmitted through the BS's backhaul link.
The computational complexity of joint reception and joint transmission algorithms increases with the number of UEs under cooperation. Consequently the hardware requirements of network nodes increase. Also the CoMP-related information exchange between cells increases with the number of UEs under cooperation. Consequently the capacity requirement of the network connecting cells increases or, with a given deployment, the probability of network overload. This is especially the case with inter-site CoMP.
In more detail, the transport links connecting the serving access node and the at least one supporting access node may not have sufficient transport link capacity for transmitting the required user data, control data and data for O&M between the serving access node and the at least one supporting access node. In case of distant access nodes, the transport link can, for example, be realized as a leased wired telephone line having a transport link capacity of about 2 Mbps, a wireless microwave link having a transport capacity of several tens of Mbps, a wired Digital Subscriber Line (DSL) connection having a transport capacity of several hundreds of Mbps, or a wired fiber connection having a transport capacity of several thousands of Mbps. In case of co-located access nodes, the transport link might, for example, be a computer bus running on the backplane of a server rack connecting two processing units. Due to the limited bandwidth of such or similar transport links, sometimes not sufficient user data, control data and data for O&M can be transported between the serving access node and the at least one supporting access node so that the access nodes cannot cooperate with each other.
Thus, for access nodes cooperating or intending to cooperate with each other in order to serve mobile stations, the problems of limited resources arise, e.g. limited transport link capacity on the network paths between the access nodes.